The present invention relates to a phase change memory device, and more particularly, to a technology for performing a write operation corresponding to a cell characteristic using a write voltage having multiple levels in a write mode.
Nonvolatile memory, including magnetic memory and phase change memory (PCM), has a data processing speed similar to that of volatile Random Access Memory (RAM) and conserves data even after power is turned off.
FIGS. 1a and 1b are diagrams illustrating a conventional phase change resistor (PCR) 4.
The PCR 4 comprises a phase change material (PCM) 2 formed between a top electrode 1 and a bottom electrode 3. A high temperature is generated in the PCM 2 when a voltage and a current are transmitted causing an electric conductive state change depending on the resistance of the PCM 2.
The PCM may include AgLnSbTe. The PCM 2 includes chalcogenide having chalcogen elements (S, Se, Te) as a main ingredient, and more specifically a germanium antimonic tellurium (Ge2Sb2Te5) consisting of Ge—Sb—Te.
FIGS. 2a and 2b are diagrams illustrating a principle of the conventional PCR 4.
As shown in FIG. 2a, the PCM 2 can be crystallized when a low current less than a threshold value flows in the PCR 4. As a result, the PCM 2 is crystallized as a low resistant material.
As shown in FIG. 2b, the PCM 2 has a temperature higher than a melting point when a high current more than a threshold value flows in the PCR 4. As a result, the PCM 2 becomes amorphous as a high resistant material.
In this way, the PCR 4 is configured to store nonvolatile data which corresponds to the two resistance states. Data “1” refers to when the PCR 4 is at a low resistance state and data “0” refers to when the PCR 4 is at a high resistance state. As a result, the logic states of the two data can be stored.
FIG. 3 is a diagram illustrating a write operation of a conventional phase change resistant cell.
Heat is generated when current flows between the top electrode 1 and the bottom electrode 3 of the PCR 4 for a given period of time. As a result, the PCM 2 is changed to a crystalline or amorphous state depending on a temperature given to the top electrode 1 and the bottom electrode 3.
When a low current flows for a given time, the PCM 2 changes to a crystalline state due to low temperature heating so that the PCR 4 which is a low resistor is at a set state. On the other hand, when a high current flows for a given time, the PCM 2 changes to an amorphous state due to high temperature heating so that the PCR 4, which is a high resistor, is at a reset state. A difference between two phases is represented by an electric resistance change.
A low voltage is applied to the PCR 4 for a long time to write the set state in a write mode. Conversely, a high voltage is applied to the PCR 4 for only a short time to write the reset state in the write mode.
However, the conventional phase change resistance cell has different characteristics determined by several process conditions. In a write mode, when data is written to all the phase change resistance cells with the same write voltage level, it is difficult to efficiently write data while considering the individual characteristics of each phase change resistance cell.